LEDs

Serial

UART

To enable uart mode, short pins at R80 or connect it to ground (see picture) and connect R65 to 3.3V (serial port vcc). Than you can send u-boot.asc to serial port.
For example if your serial port is ttyUSB0 than do:
“cat u-boot.asc > /dev/ttyUSB0” and wait until u-boot starts.